1. Field of the Invention
The present invention relates to an imaging apparatus, and a method for switching between chromatic and monochromatic modes of an imaging apparatus. More particularly, the present invention provides an imaging apparatus and a method where switching between chromatic mode and monochromatic mode is improved so as to prevent incorrect operation.
2. Description of the Related Art
A visual surveillance camera such as CCTV has a day/night monitoring function such that a filter which filters the optical image received from a lens is switched according to a brightness of a subject, and an optimized image may be obtained. A chromatic mode is selected in daytime, and a monochromatic mode is selected at night. The chromatic mode is selected by switching on Infra-Red (IR) filter, and the monochromatic mode is selected by switching off the IR filter. Switching off the IR filter means selecting a focus filter. In general, the selection of chromatic mode and monochromatic mode is performed automatically according to a brightness of a subject calculated based on AE (Auto Exposure) function.
AE function operates to get an optimized image by adjusting a diaphragm, AGC (Auto Gain Control) gain and shutter speed, that is, by adjusting minimum illumination. If a subject is darker than a predetermined standard brightness, the diaphragm is opened to receive more light, AGC gain increases to amplify a brightness value of an input image signal, and the minimum illumination increases to slow the shutter speed and to receive the light for a longer period of time so that the image can be brighter. If a subject is brighter than a predetermined standard brightness, the diaphragm, the AGC gain and the minimum illumination are oppositely adjusted so that an image can be kept at a proper brightness.
FIG. 1 is a flowchart for explaining a conventional method for automatically switching between chromatic mode and monochromatic mode. Referring to FIG. 1, a brightness level of an image is read from AE data (S100). A determination whether the present mode is a chromatic mode or a monochromatic mode is performed (S110). If the present mode is a chromatic mode, then a determination whether to switch to a monochromatic mode is performed (S120). In general, a user in advance sets a brightness level, which is a basis for switching from a chromatic mode to a monochromatic mode, and usually the brightness level is approximately 40 to 60 IRE. The IRE (Institute of Radio Engineers) is a unit for expressing a video signal magnitude, 100 IRE is a maximum white level, and 0 IRE is a black level.
Assuming, for example, a monochromatic mode reference level for switching from a chromatic mode to a monochromatic mode is set to 50 IRE. Accordingly, if the brightness level calculated at step S100 is less than 50 IRE, and the present mode is a chromatic mode as determined at step S110, a switch is made to a monochromatic mode at step S130 based on the result of determination at step S120. On the other hand, if it is determined at step S110 that the current mode is a monochromatic mode, then at step S140 it is determined if the calculated brightness level is greater than a chromatic mode reference level, which is a threshold for switching to a chromatic mode. In general, the chromatic mode reference level is approximately 90 to 110 IRE, so the chromatic mode reference level may be set to, for example, 100 IRE. Accordingly, if a brightness level of a current image is greater than 100 IRE, a switch to a chromatic mode is performed at step S150. The brightness level of an image increases or decreases to the proximity of a predetermined reference level, according to the chromatic mode switching or the monochromatic mode switching.
However, if a chromatic mode or a monochromatic mode switching time is solely based on the brightness level read from the AE data as described above, difference between the chromatic mode reference level and the monochromatic mode reference level can not increase more than a certain value, due to AE function which keeps a constant brightness.
As such, if a certain type of light source such as a halogen lamp is used, when a chromatic mode is switched to a monochromatic mode, the brightness level of an image increases to approximately 80 to 120 IRE. The increase in the image brightness level would likely cause a switching back to the chromatic mode because the increased brightness level would likely be greater than 90 to 120 IRE, which is a monochromatic mode reference level above which switching to a chromatic mode from a monochromatic mode is performed. As a result of switching to the chromatic mode, the brightness level of an image becomes less than 40 IRE, and therefore, a flashing phenomenon may occur in which the switching process (back to the monochromatic mode) is repeated.